The present invention relates to an improved static compass suitable to supply a digital indication of the magnetic head angle, of the difference angle between a selected head angle and the magnetic head angle itself. The static compass according to the present invention comprises also means for correcting the permanent magnetic fields and the induced magnetic fields.
In the following specification, only for sake of simplicity it will be assumed that the angles will be measured in integer sexagesimal degrees.
It will be evident that the principles set out are valid also in cases when a different unit will be used, or fractions of degree will be required.
In the following specification, still for sake of clearness it will be assumed that the magnetic sensor will operate as a second harmonic saturable indicator.
It will be obvious that the principles as set out are valid also in cases wherein other magnetic sensors will be used, for instance those operating as fundamental frequency saturable indicators or peak inductors.
The compass system according to the present invention provides the use of a magnetic sensor, that is conventional detector of the horizontal component of the earth magnetic field, such as for instance a conventional fluxgate. It is known that a device of this kind is energized by an alternating current signal having a f.sub.1 frequency, and supplies, if the case may be through a Scott T transformer, a frequency spectrum including two signals having the frequency 2 f.sub.1 the amplitude of which depends upon the angle .theta. between the horizontal component of the magnetic north and the magnetic axis of reference of said detector. This part of the system is commercially available and thus it will not be described in detail.
Thus, as reference axes are assumed to be three cartesian axes rigid with the sensor and the vehicle (assumed in normal trim conditions, namely with no list) whereon the sensor is fixed. The three cartesian axes are oriented as shown in FIG. 1, with their origin in the point where the sensor is located.
The magnetic field at that point is set out by the known Poisson equations: ##EQU1## where X, Y, Z are the orthogonal components of the magnetic earth field F as shown in FIG. 2.
Assuming that the sensor is of the type with two orthogonal axes (or anyway transformed to two orthogonal axes by a Scott T transformer) and that the two axes of the sensor are oriented exactly as the horizontal reference axes OX and OY, the second-harmonic components at the output of the sensor can be expressed by the two equations: ##EQU2## where .omega..sub.1 =2.pi.f.sub.1 and K is a non dimensional coefficient of proportionality depending upon the characteristics of the sensor, the possible Scott T transformer and the analogic part of the processor.
To the two values V.sub.X.sbsb.1 and V.sub.Y.sbsb.1 corresponds the angle .theta..sub.1 given by the expression: ##EQU3## with .theta..sub.1 head angle (with respect to magnetic nord) which would be indicated by the compass if no correction would be effected and with .theta. head angle with respect to the magnetic north, given by the expression ##EQU4##
The error angle in the absence of correction is given by the expression: EQU .epsilon.=.theta.-.theta..sub.1 ( 8)
The correcting circuits, once the correcting procedure has been correctly carried out, serve to annul .epsilon. and thus they transform the input signals of the equations (4) and (5) into the output signals as follows: ##EQU5##
The signal represented by the equation (10) is submitted to a phase shift of 90.degree. whereby it will be transformed into the signal as follows as: EQU V'.sub.Y =K'Y sin 2.omega..sub.1 t (11)
By adding and subtracting the (9) and the (11), two signals: sum S, and difference D will be obtained, respectively: EQU S=K'(X cos 2.omega..sub.1 t+Y sin 2.omega..sub.1 t) (12) EQU D=K'(X cos 2.omega..sub.1 t-Y sin 2.omega..sub.1 t) (13)
Keeping into account the fact that the EQU X=H cos .theta. EQU Y=-H sin .theta.
it will be possible to write for the sum and difference signals: EQU S=K'H cos (2.omega..sub.1 t+.theta.) (14) EQU D=K'H cos (2.omega..sub.1 t-.theta.) (15)
It is to be noted that the sum and difference signals S, D are two sinusoidal signals having amplitudes independent from .theta., an angular frequency 2.omega..sub.1, and phase shifted between themselves through 2.theta..
This angle 2.theta. is proportional to the time lasting from the passage through zero of the sinusoid representing S and the passage through zero of the sinusoid representing D.
Once 2.theta. is known, it will be possible to obtain the angle .theta. representing the head angle with respect to the magnetic north.
The compass system according to the present invention will be now described in detail with reference to the attached drawings.